Target closing rate showing 100+ knots too fast on TID

[Jar72]

Hello all and Heatblur team,

 

I was doing some BFM training with friends. I was assessing bandit's speed to verify he was on the target briefed speed. I checked radar on TID and the closure rate.

There was approximately 100-150 knots difference compared to what I calculated. My formula is simple : bandit speed (IAS) = closure rate - my speed (IAS).

Is my formula incorrect ? Is it something normal ? Maybe I did not read the numbers correctly ?

Ex : me = 460 kts, closing rate = 1050 kts, bandit calls on the radio he is at 460 kts. Not 590 ???

 

Thank you for your help. I did not find anything on the forum nor Internet.

JAR

 

PS : We do have 1 mod installed on our server : "range targets mod". Would that introduce a glitch possibly ?

PSbis : track file is 110 Mb, I cannot send in the forum, but I'll be happy to share a link for download.

 

Edited Sunday at 03:10 PM by Jar72

[Dragon1-1]

Closure is not based on IAS. IAS tells you about aerodynamic forces, not about how fast either of you is physically moving through the airspace. Since that's the point of the closure readout, it should be using GS.

[Jar72]

Right, but not sure the difference between ias, tas, gs would be so much, specially with no wind. We were a bit high, around 12k feet. At sea level, it should be very close (the same ?) if I remember well.

 

I ll play the replay and double check with ground speed indicated in tid in both aircraft. I ll keep you posted. But I think you are spot on. 

 

Thank you for your quick reply. It helped me moving forward on my issue.

 

 

Edited Sunday at 08:07 PM by Jar72

[Pillowcat]

[Jar72]

Hello again,

Definitely spot on @Dragon1-1. I went back to theory, and indeed, TAS (True Air Speed) depends on altitude (pressure). I totally forgot about all that. As we are doing a substraction to assess his speed, we don't mind about wind, and therefore ground speed difference will be equal to true speed difference (unless we have a different wind at each's location !!!).

Assessing the bandit speed is a bit more tricky than what I thought then. I'm sure there is a formula, the same kind of thing as for Mach depending on altitude. But I did a quick test in DCS to get rough numbers and define a rule of thumb.

My findings :

• At sea level, TAS = IAS.
• At 10 000 feet, 300 knots TAS = IAS +45 knts
• At 10 000 feet, 500 knots TAS = IAS +60 knts
•  
• At 20 000 feet, 300 knots TAS = IAS +100 knts
• At 20 000 feet, 500 knots TAS = IAS +125 knts

 

So, to evaluate bandit's speed based on closure rate, my rule of thumb will be :

• Sea level : bandit = closure - my_speed
• 10 k : bandit = closure - my_speed - 100
• 20 k : bandit = closure - my_speed - 200

 

This way, you know if bandit is slow or fast, and you can choose 1 or 2 circle.

Thanks again, it felt good to go back to "good old physics laws".

JAR

6 minutes ago, Pillowcat said:

Thanks !

[Jar72]

37 minutes ago, Pillowcat said:

Strange, I did not find the same results. I had twice less...

 

EDIT : Got it! Strange as it may, you have to change the temperature by yourself in this tool. It is not per standard atmosphere model, or so it seems.

Edited Monday at 12:21 AM by Jar72

[RustBelt]

It wouldn't be standard model. It's for finding TAS at current conditions either as recorded or as forecast. 
 

Outside of DCS a standard day is quite rare.

[Jar72]

Sorry but I kind of disagree about the standard day being rare, or about not being standard model in the t'as calculator. If you want to be purist, let me clarify what I had in mind then please. To prevent misinterpretation of the limitations, or rather non limitations in this case, of dcs.

I was referring to temperature of air varrying with altitude. When designing aircrafts for instance, you use standard atmosphere model, with a temperature gradient based on altitude. I tend to think dcs has a kind of a model for the atmosphere as well, based on the standard, with variation of the reference point depending on weather conditions. These standard models are pretty representative of real conditions, when setting the ground pressure and temp to match actual weather values.

In the calculator, if I use the standard temperature at 12 000 feet, around -24 degrees celcius, I have the same values for tas than the ones I measured in dcs, with similar weather conditions than standard. So I m happy, all seems consistent.

The screenshot above shows a temperature of 20 degrees at 12 000 feet. Therefore it explains the difference with my tests in dcs. For the fun of it, making the calculation with 20 degrees celcius at 12 000 feet means you will be in a really hot atmosphere, like 60 degrees celcius on the ground. I don t think this place exists yet.

If the calculator and dcs do work as I think now, I m happy, all is consistent. And this shows dcs is actually quite representative of what an aircraft sees in real conditions. And we surely can close the topic. If not, then huh huh. I m still missing something.

Thanks and see yah!

 

Edited Monday at 12:48 PM by Jar72

[razo+r]

30 minutes ago, Jar72 said:

Sorry but I kind of disagree about the standard day being rare. If you want to be purist, let me clarify what I had in mind then please. To prevent misinterpretation of the limitations, or rather non limitations in this case, of dcs.

I was referring to temperature of air varrying with altitude. When designing aircrafts for instance, you use standard atmosphere model, with a temperature gradient based on altitude. I tend to think dcs has a kind of a model for the atmosphere as well, based on the standard, with variation of the reference point depending on weather conditions. These standard models are pretty representative of real conditions, when setting the ground pressure and temp to match actual weather values.

In the calculator, if I use the standard temperature at 12 000 feet, around -24 degrees celcius, I have the same values for tas than the ones I measured in dcs, with similar weather conditions than standard. So I m happy, all seems consistent.

The screenshot above shows a temperature of 20 degrees at 12 000 feet. Therefore it explains the difference with my tests in dcs. For the fun of it, making the calculation with 20 degrees celcius at 12 000 feet means you will be in a really hot atmosphere, like 60 degrees celcius on the ground. I don t think this place exists yet.

If the calculator and dcs do work as I think now, I m happy, all is consistent. And this shows dcs is actually quite representative of what an aircraft sees in real conditions. And we surely can close the topic. If not, then huh huh. I m still missing something.

Thanks and see yah!

 

Standard temperature at 12'000ft will be -9 degrees (-2 degrees C per 1000ft, starting from 15 degrees C at sea level). With standard temperature gradient, if the temperature is 20 degrees at 12'000ft, that equals to 44 degrees C at sea level, and that is achievable at some places on earth.

Edited Monday at 12:50 PM by razo+r

[Jar72]

12 minutes ago, razo+r said:

Standard temperature at 12'000ft will be -9 degrees (-2 degrees C per 1000ft, starting from 15 degrees C at sea level). With standard temperature gradient, if the temperature is 20 degrees at 12'000ft, that equals to 44 degrees C at sea level, and that is achievable at some places on earth.

Oh man, you are killing me! Lol. Of course, fully agree. I googled it quickly, I don't know where the - 24 came from then !

I was in south atlantic map for the tests though... 44 degrees celcius not common there. I don t remember exact reference temperature, but still pretty similar results between dcs and the tas calculator. 

Anyway, i think I got the answers I needed, no need for me to re do accurate tests to prove dcs is rightly modelled or not. The point of this thread was to understand where the difference came from when assessing bandit ias based on closure rate. I believe we all made good points, don t you agree?

 

Thanks. Jar.

[RustBelt]

My point is, in practice you enter the actual OAT or Forecast WA temp. Even in dcs people can and do change the surface temp in missions which then changes temps aloft. So you would be plugging actual temp in that tool.